1. Field of the Invention
The present invention relates to a process of reduction of aldehydes or ketones, and more particularly, to a process of reduction which is conducted either in a gas phase or in a liquid phase by using a solid catalyst to obtain corresponding alcohols.
2. Description of the Prior Art
The Meerwein-Ponndorf-Verley reduction process is well known and has been widely used for reduction of aldehydes or ketones, to obtain corresponding alcohols.
In this process, aldehyde or ketone is subjected to reaction in the presence of metal alcoholate and alcohol. A number of combinations of metal alcoholates and alcohols have been tried, in order to attain an optimum result, of which the combination of aluminium isopropoxide and 2-propanol has proved to be most effective, due to it having the following advantages:
Firstly, the reaction using aluminium isopropoxide and 2-propanol is accompanied by hardly any adverse side reaction, such as aldol condensation.
Secondly, since aluminium isopropoxide is soluble not only in alcohol but also in hydrocarbon compounds, the reaction can be conducted in a homogeneous state.
Thirdly, since the reaction proceeds quite rapidly, a high yield of the reduction product can be realized.
Neverthless, the process is accompanied by certain disadvantages. Since the above-described known process consists in a homogeneous reaction, it requires complex and painstaking follow-up operations including hydrolysis, extraction using an organic solvent, dehydration and distillation, so as to isolate the product after the completion of the reaction.
Moreover, since the carbonyl group of the starting compound is converted to a hydroxyl group in the final product of the reaction, its hydrophilic property is significantly enhanced as compared with that of the starting compound.
Therefore, the ratio of the part of the product that can be extracted from the aqueous layer, by means of an organic solvent, to the overall product will be inevitably held at a relatively low level, resulting in a poor yield of the reaction product.
Besides, whereas active aluminium isopropoxide is of trimer type, any aluminium isopropoxide now available in the market and/or which is supplied in a crystal form is of tetramer type and has poor catalytic activity. Therefore, a catalyst to be used for this process should be prepared immediately before the initiation of the reduction process.
What is more, catalysts which can be used for this process require particular care in their storage, because during storage they can be gradually hydrolyized by water moisture in the air, and become aluminium hydroxide which has no catalytic actively.
Furthermore, in this process, catalyst which has been used should be hydrolyzed so as to isolate the product from the reaction system, upon completion of the reaction. As a result, it cannot be used further.